Process for producing powdery linear polymer having improved powder properties

ABSTRACT

A powdery linear polymer having a narrow particle size distribution with little dusting and excellent in anti-blocking property is recovered from a latex of a linear polymer containing a large proportion (35-75 wt. %) of soft polymer (S) component through a two-step coagulation process including a first (moderate coagulation) step causing 70-98 wt. % of the coagulation. The linear polymer in the latex has a multilayer structure comprising a linear polymer (S) having a glass transition temperature below 40° C. and a linear polymer (H) having a higher glass transition temperature disposed in totally at least two layers and containing 35-75 wt. %, as a whole, of the linear polymer (S) with the proviso that the linear polymer (S) cannot be contained in excess of 30 wt. % in an outermost layer of the multilayer structure.

TECHNICAL FIELD

The present invention relates to a process for recovering a powderylinear polymer having a narrow particle size distribution and anexcellent anti-blocking property from a latex of a linear polymer.

BACKGROUND ART

A polymeric molding material is subjected to molding by itself or aftermixing with another additive. For example, in the case of a vinylchloride resin, it is well known that the vinyl chloride resin issubjected to molding after powdery mixing with a polymeric modifier,such as an impact modifier comprising a rubber-based graft copolymer, afusion promoter-type processing aid comprising, e.g., an acrylic resinhaving a relatively high glass transition temperature (Tg), or alubricant-type processing aid comprising, e.g., an acrylic resin havinga relatively low Tg. Such a polymeric modifier is recovered in a powderform from a latex of the polymer, and for its use as a molding materialor for the powdery mixing thereof prior thereto, should desirably havegood powder properties as represented by a narrow particle sizedistribution, and excellent flowability and anti-blocking property.However, such desires have not been necessarily met so far.

For example, regarding the lubricant-type polymeric processing aid,multilayer polymer structures comprising a low-Tg polymer and a high-Tgpolymer and containing a relatively large proportion of the low-Tgpolymer, have been proposed (JP-B 50-37699, JP-A 49-120945, JP-A50-9653), but the powder properties thereof have not been satisfactorilyimproved.

More specifically, as for methods of obtaining powdery or particulateproducts from polymer latexes, there have been used a method of mixing apolymer latex with an electrolyte aqueous solution under stirring tocoagulate the resin content, a method of spraying a polymer latex into ahot gas stream to dry the polymer latex, etc.

However, the powdery products obtained through the above methods containa large amount of fine powder fraction, so that they are accompaniedwith many problems in handling, such as poor filterability ordewaterbility, and dusting after drying (i.e., scattering of dust duringoperations, such as transportation, metering and loading of powderyproducts) resulting in poor operation environments.

In contrast thereto, several methods including two-step coagulation ofpolymer latexes for recovering powdery or particulate products havinggood powder properties from such polymer latexes, have been proposed(JP-A 59-91103, JP-A 60-217224, JP-A 6-24009, etc.). These methods areunderstood as having succeeded to some extent in recovery of powderypolymers having improved powder properties from crosslinked rubber-basedlatexes, but it is difficult to regard that these methods have succeededas methods for recovering linear polymer powders. In contrast thereto,our research and development group has succeeded in recovering a powderypolymer having good powder properties from a latex of an acrylic linearpolymer having a relatively high glass transition temperature JP-A10-107626) but has not succeeded in recovering a powdery polymer havinggood powder properties from a latex of a linear polymer containing 30wt. % or more of polymer component having a glass transition temperaturebelow 40° C.

DISCLOSURE OF INVENTION

In view of the above-mentioned circumstances, a principal object of thepresent invention is to recover a powdery polymer which has a sharpparticle size distribution with little fine or coarse powder fraction,is little liable to cause dusting and also is excellent in anti-blockingproperty, from a latex of a linear polymer containing a large proportionof soft polymer component having a low glass transition temperature.

Thus, according to the present invention, there is provided a processfor producing a powdery linear polymer, comprising:

-   -   a step of forming a latex (A) of a multilayer polymer having a        multilayer structure comprising a linear polymer (S) having a        glass transition temperature below 40° C. and a linear        polymer (H) having a higher glass transition temperature        disposed in totally at least two layers and containing 35-75 wt.        %, as a whole, of the linear polymer (S) with the proviso that        the linear polymer (S) cannot be contained in excess of 30 wt. %        in an outermost layer of the multilayer structure,    -   a moderate coagulation step of adding a coagulant to the        latex (A) to coagulate 70-98 wt. % of the multilayer polymer,        and    -   a further coagulation step of further adding a coagulant to the        latex to complete the coagulation of the multilayer polymer.

Some history as to how we have arrived at the present invention as aresult of our study for achieving the above object, will now be brieflysupplemented.

After our success in recovery of a powdery polymer having good powderproperties through a two-step coagulation process including a moderatecoagulation step of a rubber-based graft copolymer latex (as reported inJP-A 59-72230), we have had a recognition that coagulated particleformation under the moderate coagulation conditions proceeds on adelicate balance between a heating temperature and softening of polymerparticles. More specifically, as a result of heating during thecoagulation, the melt-sticking of latex particles proceeds to formspherical enlarged coagulated polymer particles. During the step,however, the polymer is in a softened state, so that the formation offurther enlarged coarse particles and/or blocking are liable to occureasily subsequently. Such a difficulty can be alleviated by the presenceof a crosslinked trunk polymer having an anti-heat-softening property inthe graft copolymer. In the case of a latex containing uniformlystructured particles of a linear polymer, however, when the system isheated in such a degree as required for providing a certain level ofparticle size, the entire polymer is liable to be softened by the heatto easily cause the formation of coarsely large particles and/or theblocking. For obviating such difficulties, it is consequently impossibleto provide sufficient temperatures for the moderate coagulation andsubsequent heat treatment, thus resulting in particles which have a lowbulk density and are liable to be broken to finally provide a fineparticulate product containing much fine powder fraction.

As a result of further study based on the above knowledge, we succeededin two-step coagulation treatment including moderate coagulation of alinear polymer latex by using a multilayer structure including a high-Tgpolymer coated with a low-Tg polymer instead of the graft copolymer soas to charge the high-Tg polymer with the role of the rubber trunkpolymer of the graft copolymer (JP-A 10-17626). The multilayer structureadopted in the above is represented as an H/S structure in a sequencefrom the inside to the outside if the low-Tg (soft) polymer isrepresented by “S” and the high-Tg (rigid or hard) polymer isrepresented by “H”. In the multilayer structure, however, if the outsidelow-Tg polymer (S) is used in excess of 30 wt. %, the particles formedby coagulation at the moderate coagulation temperature is liable tofurther melt-stick to each other, thus causing the formation of coarselylarge particles and/or the blocking. Accordingly, at that time, we hadto give up further increasing the low-Tg polymer (S).

As a result of further study, however, it has been gradually clarifiedthat the melt-stickability-imparting effect of the linear polymer (S) inthe H/S linear polymer structure can be attained not only in the H/Sstructure but also in the S/H structure. More specifically, we assumedthat the linear polymer (S) functioning as a glue in the moderatecoagulation effectively operated only when it was in the outside, but ithas been found that the linear polymer (S) can effectively operate evenwhen it is in the inside. After all, it is understood that aheterogeneous joining state of S/H or H/S instead of a uniform mixturestructure of a linear polymer, i.e., a multilayered state retaining therespective properties of the polymer (S) and the polymer (H),effectively functions for providing a harmony between the promotion ofadhesion between adjacent particles and the prevention of blocking.Incidentally, there is a further possibility that a structure retaininggradients of the respective properties from S to H or H to S at theboundary or joint between S and H provides an effective function, whilethis has not been clarified as yet. As a result of further study, it hasbeen found possible to proceed with smooth moderate coagulation whilepreventing the excessive coagulation such as the blocking, if thepolymer (S) is not excessively localized at the surface, that is, if theamount of the polymer (S) in the outermost layer of the multilayerpolymer is suppressed to be at most 30 wt. % of the total polymer (i.e.,a part of the polymer (S) is disposed in an inner layer). Consequently,we have had a knowledge that if the above condition is satisfied, it ispossible to incorporate 35 wt. % or more of the polymer (S), and havearrived at the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

The multilayer polymer constituting the latex (A) subjected to moderatecoagulation or partial coagulation is a multilayered polymer comprisingat least one layer of linear polymer (S) having a glass transitiontemperature (Tg) below 40° C. (hereinafter also called a “soft polymer”or “polymer (S)”), and at least one layer of linear polymer (H) having ahigher Tg (hereinafter also called a “hard polymer” or “polymer (H)”).Herein, the term “linear polymer” is used in a sense of beingdifferentiated from a crosslinked polymer.

Of 100 wt.parts of linear polymer constituting the multilayer polymer,polymer component(s) having a glass transition temperature below 40° C.and therefore classified under a term “polymer (S)” are containedtotally in an amount of 35-75 wt.parts. If the content is below 35wt.parts, the multilayer polymer is free from the problem inanti-blocking property to be solved by the present invention. In excessof 75 wt.parts, it becomes difficult to obtain a powdery polymer havingan essentially good anti-blocking property.

In other words, a hard polymer (component) is necessary in order toprovide an appropriate degree of hardness to the multilayer polymer inthe latex or the powder product thereof, thereby reducing the adhesionand precipitation during the polymerization and coagulation andproviding the powder product with an improved anti-blocking property.For this purpose, the hard polymer is required to be present in anamount of at least 25 wt.parts in the multilayer polymer in the presentinvention, and if this is not satisfied, it becomes difficult to imparta sufficient rigidity at a temperature around room temperature wheresuch a powdery product is handled, thus resulting in a powdery producthaving only an inferior anti-blocking property.

Next, the structure of the multilayer polymer contained in the latex (A)will be described.

The number of layers in the multilayer polymer is not particularlylimited, but a two-layer or a three-layer structure is preferred in viewof the complexity of production and the length or number of steps forproduction of multilayer polymers having a larger number of layers. Ineither case of the two-layer or three-layer structure, it is preferredto provide a multilayer structure, including an outermost layercomprising a linear polymer having a glass transition temperature of atleast 40° C. or a linear polymer having a glass transition temperaturebelow 40° C. in an amount of 0-30 wt.parts per 100 wt.parts of themultilayer polymer.

The latex of the multilayer polymer subjected to moderate coagulationaccording to the present invention may preferably be produced througheither one of the following two process embodiments.

(First Embodiment)

The multilayer polymer latex (A) is produced through a process includingthe steps of:

-   -   (a) in the presence of 0-60 wt.parts of a linear polymer (H1)        having a glass transition temperature of at least 40° C.,        polymerizing a monomer mixture which, when polymerized alone,        provides a glass transition temperature below 40° C., to form        35-75 wt.parts of a linear polymer (S), and    -   (b) in the presence of the polymers (H1) and (S), further        polymerizing a monomer mixture which, when polymerized alone,        provides a glass transition temperature of at least 40° C., to        form 5-65 wt.parts of a linear polymer (H2) so as to proved 100        wt.parts in total of (H1)+(S)+(H2).        (Second Embodiment)

The multilayer polymer latex (A) is produced through a process includingthe steps of:

-   -   (a) in the presence of 5-75 wt.parts of a linear polymer (S1)        having a glass transition temperature below 40° C., polymerizing        a monomer mixture which, when polymerized alone, provides a        glass transition temperature of at least 40° C., to form 25-65        wt.parts of a linear polymer (H), and    -   (b) in the presence of the polymers (S1) and (H), further        polymerizing a monomer mixture which, when polymerized alone,        provides a glass transition temperature below 40° C., to form        0-30 wt.parts of a linear polymer (S2) so as to provide 100        wt.parts in total of (S1)+(H)+(S2).

The above-described two embodiments are not quite different from eachother but are based on a common concept that the two polymers (S) and(H) form a layer structure while retaining their own properties inproximity to the surface of the multilayer polymer particles in thelatex.

The soft polymer (S) (or (S1) and (S2), as the case may be), ischaracterized by a glass transition temperature Tg below 40° C. but maypreferably have a Tg in the range of −80° C. to 35° C. The hard polymer(H) (or (H1) and (H2), as the case may be), is characterized by a glasstransition temperature Tg higher than that of the soft polymer S,preferably by a Tg of at least 40° C. It is further preferred that thehard polymer (H) has a Tg higher by at least 30° C., more preferably atleast 40° C., than that of the soft polymer (S), and particularlypreferably has a Tg in the range of 40-110° C.

The amount and the layer disposition of the soft polymer layer(s) andthe hard polymer layer(s) in the multilayer polymer may affect thetemperatures for particle formation (inclusive of coagulation) and thebulk density of the resultant particles, but may generally be determinedin consideration of operability in the moderate coagulation and theanti-blocking property of the resultant powdery product.

More specifically, in order to smoothly proceed with the moderatecoagulation and ensure the anti-blocking property of the resultantpowder by hardening to some extent the surface and vicinity thereof ofthe multilayer polymer particles, it is necessary that the hard polymeris disposed as the outermost layer, or in the case where the softpolymer is disposed as the outermost layer, a portion thereof in anamount of at most 30 wt. % of the total polymer is used to form theoutermost layer and the remaining portion in excess of the 30 wt. % isdistributed to an inner layer of the multilayer polymer constituting thelatex.

Thus, by defining the layer structure of the multilayer polymer in theabove-described manner, it becomes possible to provide a good balancebetween the anti-blocking property and the moderate coagulationoperability inclusive of an appropriate moderate coagulation temperatureeven for a latex polymer containing a relatively large proportion ofsoft polymer.

The linear polymers forming the respective layers of the multilayerpolymer may comprise either homopolymers or copolymers. Any monomersproviding linear polymers may be used without a particular restriction,and examples thereof may include: methacrylate esters, acrylate esters;styrenic monomers, such as styrene, α-methylstyrene and vinyltoluene;and vinyl cyanides, such as acrylonitrile and methacrylonitrile.Particular monomers to be used may be appropriately selected dependingon the usage or function of the resultant powdery polymer product.

Also the molecular weights of the respective polymer components are notparticularly restricted and may be adjusted depending on the usage ofthe resultant polymer product by changing the species and amount ofchain transfer agents and initiators, the polymerization temperature,the manner of addition of monomer or monomer-containing solution, etc.As the chain transfer agent, alkyl mercaptans having 4 to 12 carbonatoms, such as n-octylmercaptan and n-dodecylmercaptan, may befrequently used, but these are not restrictive.

The powdery polymer having the resin structure specified by the presentinvention may for example be used as various processing aids forthermoplastic resins. For examples, if the component soft and hardcopolymers are formed as copolymers having compositions as describedbelow, it is possible to obtain a lubricant-type processing aid whereinthe relatively soft (S) polymer component functions as a lubricatingcomponent. This is however just an example and should not be construedas restricting the usage of the powdery polymer obtained by the presentinvention.

The linear polymer (S) having a glass transition temperature (Tg) below40° C. is a (co)polymer having a weight-average molecular weight of atmost 100,000, preferably at most 50,000, obtained by polymerizing amonomer (mixture) comprising 25-100 wt. % of at least one species ofmonomer selected from alkyl acrylates having an alkyl group of 1-18carbon atoms, and 0-75 wt. % of at least one species of monomer selectedfrom other vinyl monomers copolymerizable with an alkyl acrylate; andthe linear polymer (H) having a glass transition temperature (Tg) of atleast 40° C. is a (co)polymer having a weight-average molecular weightof at least 100,000, preferably at least 300,000, obtained bypolymerizing a monomer mixture comprising 35-100 wt. % of at least onespecies of monomer selected from methacrylate esters, acrylate estersand styrene monomers, and 0-65 wt. % of at least one species of monomerselected from other vinyl monomers copolymerizable therewith. This is apreferred embodiment of combination.

Incidentally, in the present specification (including Examples andComparative Examples), the glass transition temperatures Tg of(co)polymers forming the respective layers of multilayer polymers arebased on values determined based on monomer compositions according tothe following formula (Fox's formula, e.g., as disclosed in “PlasticPolymer Science and Technology” by M. D. Baijal, John Wiley & Sons; p.205 (1982)).

1/Tg=W1/Tg1+W2/Tg2+W3/Tg3+ - - - , wherein W1, W2, W3, - - - representweight fractions of monomer components 1, 2, 3, - - - , respectively,with the proviso that W1+W2+W3+ - - - =1.0; and Tg1, Tg2, Tg3, - - -represent glass transition temperatures [K] of homopolymers of monomercomponents 1, 2, 3, - - - , respectively. Some examples of homopolymerTg values are enumerated below with respect to representative monomers:

-   methyl methacrylate (MMA), Tg=105° C.=378 K,-   butyl methacrylate (BMA), Tg=20° C.=293 K,-   butyl acrylate (BA), Tg=−54° C.=219 K,-   styrene (ST), Tg=105° C.=378 K,-   acrylonitrile (AN), Tg=97° C.=370 K.

As for the polymerization process for providing the latex (A) ofmultilayer polymer used in the present invention, it is preferred toemploy ordinary emulsion polymerization using water as the dispersionmedium. As the emulsifier, it is possible to use known anionicsurfactants and nonionic surfactants alone or in combination. As thepolymerization initiator, ordinary water-soluble or oil-solubleinitiators may be used singly or so as to form a redox catalyst system.The monomer (mixture) may be added alone or in an emulsified state intothe reaction vessel.

The polymerization may be performed according to either a batch-wisepolymerization mode or a continuous polymerization mode. Thepolymerization for forming each layer can also be performed by step-wiseaddition of monomers, so as to prevent the polymer adhesion onto thereaction vessel wall or suppress the polymerization heat, and thecomposition of monomer mixture added at each time can be different. Thepolymerization process for obtaining such a multilayer polymer latex iswell known as the grow-out emulsion polymerization method and can beeasily practiced by one of ordinary skill in the art by appropriateadjustment of an emulsifier, an initiator, a monomer addition-mode, etc.

The latex particle size of the multilayer polymer is not particularlyrestricted. However, too small a latex particle size results in too higha latex viscosity which makes the handling difficult and requires alower resin concentration leading to a lower production efficiency. Onthe other hand, too large a latex particle size results in a slowerreaction and a larger amount of residual monomer. Accordingly, a latexparticle size in a range of 50-1000 nm, more preferably 50-500 nm, ispreferred.

Another example of such a process for producing a polymer latex to beprocessed according to the present invention is disclosed in JP-B50-37699.

As described above, it is important for achieving the object of thepresent invention that the multilayer polymer in the latex (A) has theabove-mentioned specific layer structure, and it is possible to obtain apowdery copolymer having a narrow particle size distribution and anexcellent anti-blocking property only when such a multilayer polymerlatex (A) is subjected to a coagulation process as described below.

A multilayer polymer latex (ordinarily having a polymer concentration onthe order of 20-60 wt. %) produced through a process as described aboveand an aqueous coagulant solution at a low concentration are mixed underan appropriate degree of stirring. As a result, with lapse of time,spherical particles are gradually grown, and the viscosity in thecoagulation system is increased. The state of increased viscosity of thecoagulation system continues for a while, and as a majority of the latexpolymer is converted into spherical particles, the viscosity of thecoagulation system is lowered to provide a so-called particulate slurry.In order to result in such a particulate slurry, it is indispensable toprovide a moderate coagulation state showing a moderate coagulationspeed when the multilayer polymer latex and the coagulant aqueoussolution are mixed. The amount of the coagulant for providing themoderate coagulation state can vary depending on the composition of themultilayer polymer latex, the species of emulsifier and initiator usedin the polymerization, the amount of electrolyte in the latex, etc., butmay be determined by measuring the amount of polymer coagulated in thefirst coagulation step (i.e., moderate or partial coagulation step).

The condition for the first moderate coagulation step should be set soas to coagulate 70-98 wt. % of the latex polymer. The coagulated polymerpercentage can be confirmed by filtrating the slurry after the moderatecoagulation through a third-class filter paper according to JIS P3801(e.g., “No. 131”, made by Toyo Roshi K.K.) and weighing the polymer onthe filter paper.

If the coagulated polymer percentage after the moderate coagulation isbelow 70 wt. %, a large amount of yet-uncoagulated latex polymer isabruptly coagulated in a subsequent complete coagulation step, wherebythe resultant particulate product is caused to have non-uniform particleshapes and is accompanied with a large amount of fine powder fraction.On the other hand, a condition giving a coagulated polymer percentageexceeding 98 wt. % is too strong a coagulation condition and results innon-uniform particle sizes giving a broad particle size distribution.

The coagulant used in the coagulation steps in the process of thepresent invention may comprise an organic acid, an inorganic acid, aninorganic salt or an organic salt. In order to prepare a coagulantsolution, these coagulants may respectively be used in a single speciesor a combination of two or more species.

Preferred examples of the inorganic acid may include: hydrochloric acid,sulfuric acid and phosphoric acid, and preferred examples of the organicacid may include: acetic acid, oxalic acid and tartaric acid,respectively, for the coagulant.

Further, examples of the inorganic salt or organic salt suitably used asthe coagulant may include electrolytes having mono-, di- or tri-valentcations. The mono-valent cation salts may include: inorganic salts, suchas sodium chloride, potassium chloride, sodium sulfate and sodiumcarbonate, and organic salts, such as sodium acetate, potassium acetate,sodium oxalate and sodium tartarate. The divaltent cation salts mayinclude: calcium chloride, magnesium chloride, and magnesium sulfate;and organic salts, such as calcium acetate and magnesium acetate.Further, the trivalent cation salts may include: aluminum sulfate, etc.

The species and amount of such a coagulant may be appropriately selectedso as to effect the partial or moderate coagulation providing acoagulated polymer percentage of 70-98 wt. % after the first coagulationstep.

The following description may be relied on only as rough standard forselection of the species and amount of the coagulant while they can varyalso depending on a slurry concentration and a coagulation temperaturedescribed hereinafter.

1) In case where the coagulant is an inorganic acid or organic acid, apH of 2.0-4.5, in the coagulation system after mixing of the polymerlatex and the coagulant aqueous solution.

2) In case where the coagulant is a mixture of an inorganic acid ororganic acid and an inorganic salt or organic salt, a higher pH isdesirable during the coagulation, that is 3.0-6.0.

3) In case where the coagulant is an inorganic salt or organic salthaving a mono-, di- or tri-valent cation, the concentration of the saltin the coagulation system is 0.08-0.5 mol/l, for the monovalent cation;0.005-0.05 mol/l, , for the divalent cation; and 0.0008-0.005 mol/l, forthe trivalent cation.

Whether the coagulant should be an inorganic acid or organic acid, or aninorganic salt or organic salt, is determined principally depending onthe species of emulsifier used in production of the multilayer polymerlatex. More specifically, in case where a carboxylic acid-typeemulsifier has been used for production of the latex, it is preferred touse an inorganic acid or organic acid as the coagulant and, in casewhere a sulfonic acid-type or nonionic-type emulsifier has been used forproduction of the latex, it is preferred to use an inorganic salt ororganic salt as the coagulant.

The multilayer polymer latex and the coagulant aqueous solution maypreferably be blended in such a ratio as to provide a slurryconcentration of 5-20% after the coagulation in order to smoothlyperform the coagulation for granulation.

At too low a slurry concentration of below 5 wt. %, the formation ofspherical particles may become insufficient to provide a powdery producthaving a broad particle size distribution including much fine powderfraction. On the other hand, at a concentration in excess of 20 wt. %,an excessive increase in viscosity of the coagulation system is liableto occur, thus making it difficult to obtain a uniform stirring stateand resulting in an undesirably broad particle size distribution.

The temperature for coagulation may be determined in view of theobjective particle size and particle size distribution and maypreferably be selected in a range of 20-100° C. Below 20° C., it becomesdifficult to effect cooling with ordinary industrial water and a specialcooling apparatus becomes necessary. Even at a temperature exceeding100° C., the coagulation can be effected by using a pressurizedcoagulation system, which however requires a special facility and also alarger energy cost. Accordingly, the coagulation temperature shoulddesirably be not exceeding 100° C., and more preferably in a range of30-85° C.

The coagulation temperature can depend on the coagulant concentrationand the stirring condition but may most remarkably be affected by theproperties of the multilayer polymer in the latex. Accordingly, in orderto smoothly effect the moderate coagulation, it is important for themultilayer polymer in the latex satisfies the above-mentionedappropriate layer structure. Generally describing about the relationshipbetween the coagulation temperature and the multilayer polymer structureor the polymer Tg's forming the respective layers, a lower coagulationtemperature is used at lower Tg of the polymers (S) and (H).

A lower coagulation temperature is also used at a larger content of thepolymer (S), and as the location of the polymer (S) approaches thesurface of the multilayer polymer. In order to determine an optimumcoagulation temperature for a latex of multilayer polymer having acertain layer structure, it is desirable to perform a preliminary testby using a temporarily set coagulation temperature which may be lower by20° C. than Tg of a polymer (H) if the polymer (H) forms the outermostlayer of the latex polymer particles or higher by 20° C. than Tg of apolymer (S) if the polymer (S) forms the outermost layer. Based on acoagulated particle size measured as a result of the preliminary test, acoagulation temperature adjustment may preferably be performed such thata higher coagulation temperature may be used when the measured particlesize is smaller than the objective particle size, and a lowercoagulation temperature may be used when the measured particle size islarger than the objective particle size.

Incidentally, the particle size and its distribution of a product powderpolymer finally obtained after drying are substantially determined bythose of the polymer particles in the slurry after the moderatecoagulation step. Accordingly, the moderate coagulation conditionsincluding the coagulation temperature may preferably be determined basedon the results of, e.g., preliminary tests as mentioned above so as toprovide an average particle size (=d₅₀, described later) in a range of80-500 μm, more preferably 100-300 μm; and a particle size dispersionfactor (=d₅₀/d₈₄, described later) in a range of 1.3-2.1, morepreferably 1.4-1.9, with respect to a finally obtained powdery polymerproduct. It is further preferred that the coagulation conditions are setso as to provide a powdery polymer product which is substantially freefrom particles having a particle size exceeding 850 μm, contains at most2 wt. %, more preferably at most 1 wt. %, if any, of particles having aparticle size below 45 μm, and has a bulk density of at least 0.34 g/cm³and a blocking strength of at most 1.0 kg.

After the moderate coagulation, it is necessary to effect a second-stepcoagulation by using an additional coagulant to complete thecoagulation. An appropriate amount of coagulant used in the secondcoagulation step can be determined from a state of completelydissipating the yet-uncoagulated polymer latex. The coagulant used inthe second-step coagulation for completely coagulating theyet-uncoagulated polymer latex need not be identical to the one used inthe moderate coagulation step, but another species coagulant can be usedif it provides a stronger coagulation condition than the moderatecoagulation to complete the coagulation.

After completion of the particle formation by dissipating theyet-uncoagulated latex, it is possible to neutralize the slurry, asdesired, by adding an acid, such as hydrochloric acid, or by adding analkali, such as sodium hydroxide, as the case may be. It is alsopossible to add, e.g., some agents for providing the powder propertiesof the coagulated particulate product at any stage during thecoagulation process. It is preferred to apply a heat treatment to theslurry after the coagulation in order to enhance the coagulation of thecoagulated particulate product, thereby increasing the bulk density ofthe powdery product after drying. The heat treatment temperature may bedetermined in a range of not causing melt-sticking of the particulateproduct formed after the coagulation, e.g., in a range of 50-100° C.More specifically, a rough standard may be given as a range of Tg ofpolymer (H) of the multilayer polymer±10° C.

The coagulation operation in the present invention may be performedeither batchwise or continuously. In the batchwise scheme, all theoperations may be performed in a single coagulation vessel, or theslurry after completion of the coagulation may be transferred to anotherstirring vessel where subsequent operations, such as the neutralizationand the heat-treatment may be performed. Further, the continuous schemeoperation may be effected in a plurality of stirring vessels disposed inseries so that the moderate coagulation is performed in the firstvessel, the second vessel is used for completing the coagulation, andthe third and subsequent vessels may be used for the neutralization, theheat-treatment, etc. The slurry after the heat-treatment may besubjected to ordinary post-treatments, such as dewatering and drying, torecover a powdery polymer product.

EXAMPLES

Hereinbelow, the present invention will be described more specificallywith reference to Examples and Comparative Examples, which howevershould not be construed to restrict the scope of the present inventionin any way. Some physical properties disclosed in Examples were measuredaccording to the following methods.

(1) Average Particle Size (Diameter) of Latex Particles.

Measured by using a sub-micron particle size analyzer (“Coulter CounterN4SD”, available from Coulter Electronics Inc.).

(2) Average Particle Size of Powdery Polymer.

20 g of a powdery sample mixed with 0.2 g of carbon black for staticelectricity prevention is placed on a stacked series of standard sievesaccording to JIS Z8801 including sieves having mesh openings of 850 μm,500 μm, 355 μm, 300 μm, 250 μm, 212 μm, 150 μm, 106 μm and 45 μm stackedin this order from the top to the bottom, followed by application ofexternal electromagnetic vibration for 10 min., to measure the amountsof powder on the respective sieves.

From the amounts of the powder on the respective sieves, a cumulativeparticle size distribution curve (cumulative amount vs. mesh opening) isdrawn, and a particle size giving a cumulative amount of 50 wt. % on thecurve is taken as the average particle size d₅₀ (μm).

Further, a particle size (=d₈₄ μm) giving a cumulative amount of 84 wt.% as counted from a large particle size side to a small particle sizeside is taken on the distributions curve to calculate a particle sizedistribution factor d₅₀/d₈₄ as a measure of particle size distributionfactor. A smaller d₅₀/d₈₄ value represents a narrower particle sizedistribution.

(3) Weight-average Molecular Weight (Mw) of a (Co)polymer.

0.05 g of a (co)polymer sample is dissolved in 5 cc of tetrahydrofuran,and the resultant sample solution is subjected to gel permeationchromatography (by using an apparatus system (“LC-6A”, made by ShimadzuSeisakusho K.K.) equipped with a column (“SHODEX KF-806L”, made by ShowaDenko K.K.), thereby obtaining a weight-average molecular weight (Mw)based on polystyrene standard examples.

(4) Bulk Density of Powdery Polymer.

Measured by using a bulk specific gravity meter according to JIS-K6721.

(5) Dusting

During the bulk density measurement, the state of falling-down of apowdery sample is observed with eyes and rated into the following ranks:

-   -   A: Free from dusting,    -   B: Slight dusting,    -   C: Conspicuous dusting.        (6) Anti-blocking Property (Blocking Strength)

0.5 g of a powder polymer is placed in a tablet molding machine andmolded into a tablet of 1 cm² in sectional area under application of apressure of 1.96 MPa for 2 hours in a thermostat vessel regulated at 35°C. Then, gradually increasing loads are applied to the tablet sample bya hardness meter (of Kiya-type) to measure a blocking strength (aminimum load (in Kg) required for breaking the tablet). The results maybe evaluated based on the measured blocking strengths roughly asfollows:

-   0-1 Kg: Excellent anti-blocking property.-   1-2 Kg: Slightly inferior anti-blocking property.-   Above 2 Kg: Much inferior anti-blocking property.

Example 1

(Production of Multilayer Polymer Latex)

Into a reaction vessel equipped with a stirrer, 0.1 wt.part oftetrasodium pyrophosphate, 0.002 wt.part of ferrous sulfate, 0.003wt.part of disodium ethylenediaminetetraacetate, 6.5 wt.parts of15.5%-potassium oleate (K.OL) aqueous solution and 200 wt.parts ofdeionized water, were charged, and then the system was aerated withnitrogen and heated to 50° C. Into the reaction vessel, Monomer mixture(1) of 36 wt.parts of styrene (ST) and 24 wt.parts of butyl acrylate(BA), 1 wt. part of n-octylmercaptan, and 0.36 wt.part each of t-butylhydroperoxide and sodium formaldehyde sulfoxylate, were added andsubjected to 3 hours of emulsion polymerization at 50° C. to obtain asoft copolymer (S) latex (a). (Incidentally, in all Examples andComparative Examples described herein, each monomer mixture was addedtogether with corresponding amounts (0.6 wt.part each per 100 wt.partsof the monomer mixture) of t-butyl peroxide (TBPO) and sodiumformaldehyde sulfoxylate (SFAS) as a polymerization initiator system,and therefore, the description of these components will be omittedhereinafter.)

In the presence of the copolymer latex (a), Monomer mixture (2) of 38wt.parts of methyl methacrylate (MMA) and 2 wt.parts of butyl acrylate(BA) (together with corresponding amounts of TBPO and SFAS) was addedand subjected to 3 hours of second-layer emulsion polymerization at 50°C. to obtain a soft/hard (S/H) copolymer latex (b).

Incidentally, Monomer mixture (1) provided a copolymer having Mw=3×10⁴,and Monomer mixture (2) provided a copolymer having Mw=3×10⁵,respectively, when polymerized separately.

The composition of the respective monomer mixtures and the resultantmultilayer polymer are summarized in Table 1 together with those ofExamples and Comparative Examples described hereinafter.

(Coagulation of Copolymer Latex (b))

Into a coagulation vessel equipped with a stirrer, 600 wt.parts of0.1%-hydrochloric acid aqueous solution (Coagulant (I)) was charged andheated to 80° C. Then, 314 wt.parts (100 wt.parts as resin) of theabove-prepared S/H copolymer latex (b) was charged to the vessel, toeffect moderate coagulation. (Incidentally, the coagulation temperatureof 80° C. was determined through a preliminary test so as to provide aparticle size of 100-200 μm with respect to a powdery product recoveredafter the coagulation, similarly as in Examples described hereinafter.)

The coagulation percentage (%), i.e., coagulated polymer percentage, atthis time, was 90% as shown in Table 3 together with those of(Comparative) Examples described hereafter.

Then, 100 wt.parts of 2%-hydrochloric acid aqueous solution (Coagulant(II)) was added to the coagulation vessel to complete the coagulation.After the coagulation, the system was neutralized with sodium hydroxide,and the resultant slurry was heated to 90° C. for heat treatment. Thethus-treated slurry was then subjected to filtration, washing withwater, de-watering and drying to recover Powder polymer (A).

Some representative properties of Powdery polymer (A) thus obtained andsome representative coagulation conditions are summarized in Table 3together with those of the following (Comparative) Examples (some ofwhich are shown in Table 4).

Example 2

(Production of Multilayer Polymer Latex)

Emulsion polymerization was performed first in two steps similarly as inExample 1 except that the emulsifier was changed to 5 wt.parts of20%-sodium laurylsulfate (NaLS) aqueous solution, Monomer mixture (1)was changed to a mixture of 18 wt.parts of methyl methacrylate (MMA) and2 wt.parts of styrene (ST), Monomer mixture (2) was changed to a mixtureof 30 wt.parts of styrene (ST) and 20 wt.parts of butyl acrylate (BA),and the n-octylmercaptan was omitted.

In the resultant copolymer latex, Monomer mixture (3) of 22.5 wt.partsof styrene (ST) and 7.5 wt.parts of acrylonitrile (AN) (together withcorresponding amounts of TBPO and SFAS) was added to effect 3 hours ofthird-layer emulsion polymerization at 50° C. to obtain an H/S/Hcopolymer latex (c).

(Coagulation)

The copolymer latex (c) was then subjected to two steps of coagulationin a similar manner as in Example 1 but under different conditions asshown in Table 3, and the resultant slurry was subjected to similar posttreatments as in Example 1 except for omitting the neutralization toobtain Powdery polymer (B).

As shown in Table 3, Powdery polymers (A) and (B) prepared in the aboveExamples 1 and 2 respectively exhibited a narrow particle sizedistribution with extremely little fine powder fraction and an excellentanti-blocking property.

Comparative Examples 1 and 2

The copolymer latex (c) prepared in Example 2 was coagulated in similarmanners as in Example 2 but two different first-step moderatecoagulation conditions shown in Table 3, i.e., at a lower coagulantconcentration (Comparative Example 1) and a higher coagulantconcentration (Comparative Example 2), thereby obtaining Powder polymers(C) and (D), respectively. The properties of these products are shown inTable 3.

As is understood from Table 3, Powdery polymer (C) exhibited a poorparticle size distribution including much coarse powder fraction andfine powder fraction because a large amount of yet-uncoagulated polymerremained after the first-step coagulation and was abruptly coagulated byaddition of further coagulant in the second coagulation step. On theother hand, Powdery polymer (D) included an excessively large amount ofcoarse powder fraction (≧850 μm). These results show that the control ofcoagulated polymer percentage in the first coagulation step in anappropriate range is essential for providing a powdery polymer having anexcellent particle size distribution.

Example 3

Three-step emulsion polymerization was performed in the same manner asin Example 2 except that the emulsifier was changed to 5 wt.parts of30%-sodium N-lauroylsarcocinate (NaNLS) and Monomer mixtures (1)-(3) forthe respective layers were changed as shown in Table 2, therebyobtaining an S/H/S copolymer latex (d). Incidentally, Monomer mixtures(1) and (3) provided copolymers of Mw≦5×10⁴, and Monomer mixture (2)provided a copolymer of Mw≧3×10⁵, respectively, when polymerizedseparately.

The copolymer latex (d) was subjected to two-step coagulation in thesame manner as in Example 2 except for changing the coagulationconditions including the species and concentration of the coagulant inthe first coagulation step as shown in Table 3, to finally obtainPowdery polymer (E).

Comparative Example 3

Two-step emulsion polymerization was performed in the same manner as inExample 1 except that the emulsifier was changed to 5 wt.parts of30%-sodium N-lauroylsarcocinate (NaNLS), Monomer mixtures (1) and (2)were changed as shown in Table 2 and the n-octylmercaptan was omitted toobtain an H/S copolymer latex (e). The latex was then subjected totwo-step coagulation in the same manner as in Example 1 except that thecoagulation conditions were changed as shown in Table 3 including thecoagulant, and the slurry was post-treated in the same manner as inExample 1 except for a lower heat-treatment temperature of 80° C.,thereby obtaining Powdery polymer (F).

Powder polymer (F) contained a substantial proportion of coarsely largeparticles (≧850 μm) due to a large proportion of S-polymer in theoutermost layer. Further, as it was impossible to impart a sufficientlyhigh heat-treatment temperature, Powdery polymer (F) also exhibited alower bulk density and inferior anti-blocking property.

Example 4 and Comparative Examples 4-6

Copolymer latexes (f)-(i) were prepared in the same manner as in Example2 except that Monomer mixtures (1)-(3) were respectively changed asshown in Table 2, and each of Monomer mixtures (1) and (3) was usedafter being mixed with 1.7 wt.parts of n-octylmercaptan per 100 wt.partsthereof. Incidentally, for each of copolymer latexes (f)-(i), Monomermixtures (1) and (3) provided a copolymer of Mw≦5×10⁴, and Monomermixture (2) provided a copolymer of Mw≧3×10⁵, respectively, whenpolymerized separately.

These latexes (f)-(i) were respectively subjected to two-stepcoagulation and post-treatments in the same manner as in Example 2except for changing the coagulation conditions as shown in Table 4 andchanging the heat-treatment temperatures to 60° C., 80° C. and 80° C.for Comparative Examples 4, 5 and 6, respectively, thereby obtainingPowdery polymers (G)-(J) (Example 4 and Comparative Examples 4-6).

Powdery polymers (H) and (I) prepared in Comparative Examples 4 and 5obtained from multilayer polymers having a large amount of S-polymer inthe outermost layer similarly as in Comparative Example 3 containedsubstantial amounts of coarsely large particles (≧850 μm), due toinadequacy of the first-step coagulation conditions.

Reference Example

Three-step emulsion polymerization was performed in the same manner asin Example 2 except that Monomers (1) to (3) were changed as shown inTable 2, i.e., so as to have identical compositions (MMA/BA=3/1) eachproviding an H-polymer (of Tg=47° C.) to obtain a copolymer latex (j) ofa multilayer polymer including three layers of identical composition.The latex (j) was then subjected to two-step coagulation andpost-treatment in the same manner as in Example 2 except for changingthe coagulation conditions as shown in Table 4 and the heat-treatmenttemperature to 65° C., to obtain Powdery polymer (K). In the case ofsuch a latex polymer having a uniform composition structure, the latexpolymer particles were softened entirely by the heating for coagulation,thus resulting in coarse particles if it was tried to obtain anobjective average particle size. Further, at a high heat-treatmenttemperature, coarsely large particles were further formed to causeblocking, so that it was impossible to impart a sufficiently highheat-treatment temperature. As a result, Powdery polymer (K) exhibited alow bulk density while it contained a large proportion of coarseparticles (>850 μm), and also finally contained a large proportion offine particles (<45 μm) due to fragile particles thereof.

The above-mentioned powder properties of the powdery polymers obtainedin the above-described Examples and Comparative Examples are inclusivelyshown in Tables 3 and 4. TABLE 1 Multilayer polymer composition((H/)/S/H) Monomer mixture *1 H//S//H*3 Example (1) (2) (3) wt. partsEx. 1 Composition ST/BA MMA/BA  0//60//40 wt. parts 36/24 38/2 Tg (°C.)*2 20 92 Ex. 2 & Composition MMA/ST ST/BA ST/AN 20//50//30 Comp. Ex.wt. parts 18/2 30/20 22.5/7.5 1, 2 Tg (° C.)*2 105 20 103Note to Table 1 (as well as Table 2 on the next page)*1: The following abbreviations are used for expressing respectivemonomers: MMA = methyl methacrylate, BMA = butyl methacrylate, BA =butyl acrylate, ST = styrene, AN = acrylonitrile.*2: Tg: Glass transition temperature obtained by separately polymerizinga monomer mixture for each layer (calculated according to Fox's formula)*3: Layers forming the multilayer polymer are indicated sequentiallyfrom the inside to the outside while using symbols, S for representing alayer of soft polymer (of Tg < 40° C.) and H for representing a layer ofhard polymer (of Tg ≧ 40° C.).

TABLE 2 Multilayer polymer composition ((S/)H/S) Exam- Monomer mixture*1 S//H//S *3 ple (1) (2) (3) wt. parts Ex. 3 Compo- ST/BA MMA/ST ST/BA20//60//20 sition wt. parts 12/8   54/6 12/8 Tg(° C.) *2 20 105 20 Comp.Compo- MMA/BA MMA/BMA/  0//50//50 Ex. 3 sition BA wt. parts 45/5  20/10/20 Tg(° C.) *2 79  7 Ex. 4 Compo- ST/BA MMA/BA ST/BA 60//30//10sition wt. parts 36/24   27/3  6/4 Tg(° C.) *2 20  92 20 Comp. Compo-ST/BA MMA/BA ST/BA  5//20//75 Ex. 4 sition wt. parts  3/2   18/2 45/30Tg(° C.) *2 20  92 20 Comp. Compo- ST/BA MMA/BA ST/BA  5//45//50 Ex. 5sition wt. parts  3/2 40.5/4.5 30/20 Tg(° C.) *2 20  92 20 Comp. Compo-ST/BA MMA/BA ST/BA 70//20//10 Ex. 6 sition wt. parts 42/28   18/2  6/4Tg(° C.) *2 20  92 20 Refer- Compo- MMA/BA MMA/BA MMA/BA  H//H//H encesition 20//40//40 Exam- wt. parts 15/5   30/10 30/10 ple Tg(° C.) *2 47 47 47Notes:*1, *2 and *3: The same as in Table 1.

TABLE 3 Conditions for Production and Coagulation of Multilayer polymerlatexes, and Properties of Powdery polymer products Example Ex. 1 Ex. 2Comp. 1 Comp. 2 Ex. 3 Comp. 3 Multilayer polymer latex (b) (c) (c) (c)(d) (e) (Layer structure) (S/H) (H/S/H) (H/S/H) (H/S/H) (S/H/S) (H/S)Emulsifier *1 K.OL NaLS → → NaNLS → Latex particle size (nm) 160 nm 140→ → 210 → Coagulation conditions 1st. Step coagulant (I) HCl CaCl₂ → →HCl HCl coagulant conc. (wt. %) 0.10 0.22 0.05 0.30 0.40 0.45 Temp(° C.)80 75 → → 55 35 coagulated polymer (%) 90 92 63 99 85 90 2nd. Stepcoagulant (II) HCl CaCl₂ → → CaCl₂ CaCl₂ coagulant conc. (wt. %) 2 1 1 11 1 Powdery Polymer (A) (B) (C) (D) (E) (F) Properties d₅₀(μm) 140 135120 170 140 125 >850 μm(%) 0 0 3.8 10 0 3.2 <45 μm(%) 0.6 0.4 8.0 5.10.3 3.5 d₅₀/d₈₄(−) 1.5 1.5 2.2 2.5 1.5 1.8 Dusting A A C B A B Bulkdensity(g/cm³) 0.35 0.37 0.31 0.36 0.39 0.31 Blocking strength (kg) 0.80.4 0.5 0.4 0.3 ≧2Notes to Table 3 (as well as Table 4 on the next page)*1: K.OL = potassium oleate, NaLS = sodium laurylsulfate, NaNLS = sodiumN-lauroylsarcocinate“→” means the same as in the adjacent left column.

TABLE 4 Conditions for Production and Coagulation of Multilayer polymerlatexes, and Properties of Powdery polymer products Example ReferenceEx. 4 Comp. 4 Comp. 5 Comp. 6 Example Multilayer polymer latex (f) (g)(h) (i) (j) (Layer structure) (S/H/S) (S/H/S) (S/H/S) (S/H/S) (H)Emulsifier*1 NaLS → → → → Latex particle size (nm) 145 → → → →Coagulation conditions 1st. Step coagulant (I) CaCl₂ → → → → coagulantconc. (wt. %) 0.28 0.32 0.31 0.29 0.32 Temp(° C.) 65 30 35 40 55coagulated polymer (%) 90 93 92 85 85 2nd. Step coagulant (II) CaCl₂ → →→ → coagulant conc. (wt. %) 1 → → → → Powdery Polymer (G) (H) (I) (J)(K) Properties d₅₀(μm) 125 145 130 142 102 >850 μm(%) 0 4.5 3.7 1.5 2.2<45 μm(%) 0.4 2.5 4.5 4.1 13.5 d₅₀/d₈₄(−) 1.5 1.9 1.9 1.8 2.0 Dusting AB B B C Bulk density(g/cm³) 0.39 0.31 0.33 0.37 0.29 Blocking strength(kg) 0.8 ≧2 ≧2 ≧2 1.3Notes:*1 and “→” are the same as in Table 3.[Industrial Applicability]

As is clear in view of the results shown in Tables 1 to 4, according tothe process of the present invention, wherein a latex of multilayerpolymer having a specific layer structure is subjected a two-stepcoagulation process including a first moderate coagulation step, itbecomes possible to smoothly proceed with moderate coagulation of alatex of a linear polymer containing a large proportion of soft polymer,thereby providing a powdery polymer product which contains little finepowder fraction liable to cause dusting, has a narrow particle sizedistribution and is excellent in anti-blocking property.

1. A process for producing a powdery linear polymer, comprising: a stepof forming a latex (A) of a multilayer polymer having a multilayerstructure comprising a linear polymer (S) having a glass transitiontemperature below 40° C. and a linear polymer (H) having a higher glasstransition temperature disposed in totally at least two layers andcontaining 35-75 wt. %, as a whole, of the linear polymer (S) with theproviso that the linear polymer (S) cannot be contained in excess of 30wt. % in an outermost layer of the multilayer structure, a moderatecoagulation step of adding a coagulant to the latex (A) to coagulate70-98 wt. % of the multilayer polymer, and a further coagulation step offurther adding a coagulant to the latex to complete the coagulation ofthe multilayer polymer.
 2. A process according to claim 1, wherein thelatex (A) is produced through a process including the steps of: (a) inthe presence of 0-60 wt.parts of a linear polymer (H1) having a glasstransition temperature of at least 40° C., polymerizing a monomermixture which, when polymerized alone, provides a glass transitiontemperature below 40° C., to form 35-75 wt.parts of a linear polymer(S), and (b) in the presence of the polymers (H1) and (S), furtherpolymerizing a monomer mixture which, when polymerized alone, provides aglass transition temperature of at least 40° C., to form 5-65 wt.partsof a linear polymer (H2) so as to provide 100 wt.parts in total of(H1)+(S)+(H2).
 3. A process according to claim 1, wherein the latex (A)is produced through a process including the steps of: (a) in thepresence of 5-75 wt.parts of a linear polymer (S1) having a glasstransition temperature below 40° C., polymerizing a monomer mixturewhich, when polymerized alone, provides a glass transition temperatureof at least 40° C., to form 25-65 wt.,parts of a linear polymer (H), and(b) in the presence of the polymers (S1) and (H), further polymerizing amonomer mixture which, when polymerized alone, provides a glasstransition temperature below 40° C., to form 0-30 wt.parts of a linearpolymer (S2) so as to provide 100 wt.parts in total of (S1)+(H)+(S2). 4.A process according to claim 1, wherein the linear polymer (S) has aglass transition temperature of −80° C. to 35° C., and the linearpolymer (H) has a glass transition temperature which is higher by atleast 30° C. than that of the linear polymer (S).
 5. A process accordingto claim 1, wherein the linear polymer (S) having a glass transitiontemperature (Tg) below 40° C. is a (co)polymer having a weight-averagemolecular weight of at most 100,000 obtained by polymerizing a monomer(mixture) comprising 25-100 wt. % of at least one species of monomerselected from alkyl acrylates an alkyl group of 1-18 carbon atoms, andat least one species of monomer selected from other vinyl monomerscopolymerizable with an alkyl acrylate; and the linear polymer (H)having a glass transition temperature (Tg) of at least 40° C. is a(co)polymer having a weight-average molecular weight of at least 100,000obtained by polymerizing a monomer (mixture) comprising 35-100 wt. % ofat least one species of monomer selected from methacrylate esters,acrylate esters and styrenic monomers, and 0-65 wt. % of at least onespecies of monomer selected from other vinyl monomers copolymerizabletherewith.
 6. A process according to claim 5, wherein the linear polymer(S) has a weight-average molecular weight of at most 5×10⁴, and thelinear polymer (H) has a weight-average molecular weight of at least3×10⁵.
 7. A process according to claim 1, further including a step ofheat-treating the coagulated multilayer polymer at a temperature whichis in a range of ±10° C. with respect to the glass transitiontemperature of the linear polymer (H).
 8. A powdery linear polymerobtainable through a process according to claim 1, wherein the powderylinear polymer has a multilayer structure comprising a linear polymer(S) having a glass transition temperature below 40° C. and a linearpolymer (H) having a higher glass transition temperature disposed intotally at least two layers and containing 35-75 wt. %, as a whole, ofthe linear polymer (S) with the proviso that the linear polymer (S)cannot be contained in excess of 30 wt. % in an outermost layer of themultilayer structure, and the powdery linear polymer has powderproperties including an average particle size d₅₀ in a range of 80-500μm and a particle size distribution factor d₅₀/d₈₄ in a range of1.3-2.1, wherein d₅₀ and d₈₄ are particle sizes giving cumulatively 50wt. % and 80 wt. %, respectively, of particles counted from a largerparticle size side to a smaller particle size side on a particle sizedistribution curve.
 9. A powdery linear polymer according to claim 8,which is substantially free from particles larger than 850 μm andcontains less than 2 wt. % of particles smaller than 45 μm.
 10. Apowdery linear polymer according to claim 8, which has a bulk density ofat least 0.34 g/cm³.
 11. A powdery linear polymer according to claim 8,which exhibits a blocking strength of at most 1.0 kg.